[sdiy] EMS VCS3 II negative power supply problems

Magnus Danielson magnus at rubidium.se
Mon Sep 19 15:27:48 CEST 2022

Hej Mattias,

It's a classic over-current protection setup. So if there is too much 
current it will make the driver back off. If this is broken you god that 
happening you the supply.


On 2022-09-16 23:53, Mattias Rickardsson wrote:
> Hej Magnus,
> yes, the power transistors are off board, and I've replaced that one. 
> Also the nearby metal can transistor. There's also a third transistor 
> in regular small plastic package close to them that I'm not completely 
> sure at the moment what it does in the negative power circuit...?
> /mr
> Den fre 16 sep. 2022 21:57Magnus Danielson <magnus at rubidium.se> skrev:
>     Hi,
>     Check the buffer transistor. That had blown on mine.
>     The Mark II PSU is nicer than the Mark I PSU for VCS3/Synthi-A.
>     About the same time as the Mark I and Mark II shift, they moved
>     the transistors off the PSU board. If they are on the board, move
>     them out as this remove heat and it will eventually fail. Guess
>     how I know.
>     Cheers,
>     Magnus
>     On 2022-09-13 21:50, Mattias Rickardsson wrote:
>>     Hey,
>>     I'm trying to revive a VCS3 II where the power supply has issues
>>     on the negative regulation. With an incoming -25 V it puts out
>>     roughly -24 V instead of the supposed -9 V. Does anyone have
>>     experience with similar problems?
>>     (The transformer was fried on the negative half, so I'm running
>>     it from a bench supply. I have also replaced the regulating 741
>>     and TIP3055 power transistor)
>>     ((The phutney.com <http://phutney.com> site and its very readable
>>     schematics also magically disappeared, which didn't help)) :-)
>>     Best,
>>     /mr
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